The response of normal tissues to multiple doses of radiation will be quantitated with a view to understanding differences in response to various tissues to the daily doses used in clinical radiotherapy. The tissue cells to be studied are bone marrow stem cells, using an endogenous spleen colony assay, hair follicle cells using a hair counting technique and renal tubule cells using a cloning technique. Doses of the order of 50 to 500 rad will be given in each dose fraction and the effect scored at appropriate intervals after exposure. It is anticipated that small differences in response at low doses will be amplified by repeated exposures, allowing more precise quantitation.